Wireless communication standards, such as GSM (Global System for Mobile Communication), EDGE (Enhanced Data Rates through GSM Evolution), and WCDMA (Wideband Code Division Multiple Access), require a variety of digital modulation formats like 4-PSK, 8-PSK (Phase Shift Keying), and various QAM implementations (Quadrature Amplitude Modulation). A general trend of evolution in the field of wireless communication is an aim for increased data rates (bits/second) and increased spectral efficiency (bits/Hertz).
Using a traditional linear modulation method, such as a traditional Cartesian QAM, imposes stringent linearity requirements for a power amplifier of a transmitter. Biasing the power amplifier to linear operating points may satisfy said linearity requirements, but does so at the expense of power efficiency of the power amplifier, i.e. power losses in the amplifier are increased.
Polar transmitters employing polar modulation methods are alternatives to transmitters employing conventional linear modulation methods. In a polar modulation method a complex valued base-band signal to be modulated up to an RF-band (Radio Frequency) is represented using polar co-ordinates, amplitude and phase, instead of using Cartesian in-phase and quadrature co-ordinates. A significant advantage of polar modulation is the fact that it can be used when using a high efficiency power amplifier such as a Class-C, Class-E, Class-F, or saturated Class-B power amplifier. This is because such amplifiers are highly non-linear and cannot pass signals that have amplitude modulation present without severe spectral re-growth and distortion. Thus an input signal of the power amplifier must have phase modulation only and no amplitude modulation, i.e. RMS-power (Root Mean Square) of said input signal must be sufficiently constant. Amplitude modulation is then applied to an output signal in a power supply of the power amplifier.
A generic concept for a polar modulator is illustrated in FIG. 1. The polar modulator 100 has two input signals: a base-band amplitude signal 101 and a base-band phase signal 102. Amplitude information and phase information are both included in an input data signal 109. Said base-band amplitude signal and base-band phase signal are formed from said input data signal 109 with base-band processing means 120. The base-band phase signal 102 is an input signal of a phase modulator 110. The input data signal, the base-band amplitude signal, and the base-band phase signal are functions of time (t). The phase modulator can be e.g. a voltage-controlled oscillator an input voltage of which is proportional to a time derivative of the base-band phase signal 102. An output signal 103 of the phase modulator 110 is a phase modulated signal an instantaneous phase of which depends on the base-band phase signal 102. The phase modulated signal 103 can be modelled with the following equation:Ph(t)=A sin(2πfct+Φ(t)),  (1)where Ph(t) denotes the phase modulated signal 103 as a function of time, A is a constant amplitude of the phase modulated signal 103, Φ(t) denotes an instantaneous value of the base-band phase signal 102 as a function of time, and fc is a center frequency of the phase modulated signal 103. Information of the base-band phase signal is included in an instantaneous phase of the phase modulated signal. Without loosing generality the amplitude A can be chosen to be unity (=1). The center frequency fc can be a final carrier frequency of a transmitter or it can be an intermediate frequency.
The output signal 103 is multiplied by the base-band amplitude signal 101 in an amplitude modulator 111. An output signal of the amplitude modulator is a polar modulated signal 104. The polar modulated signal can be modelled as:P(t)=R(t)sin(2πfct+Φ(t)),  (2)where P(t) denotes the polar modulated signal 104 as a function of time and R(t) denotes an instantaneous value of the base-band amplitude signal 101 as a function of time. If a dc-component of the base-band amplitude signal 101 is substantially zero the amplitude modulator produces a polar modulated signal in which the center frequency is suppressed. An advantage of having the center frequency suppressed is the fact that a smaller power is required to reach a desired signal-to-noise ratio in a communication path. A significant disadvantage is the fact that when the base-band amplitude signal is allowed to change its sign an instantaneous phase of the polar modulated signal depends not only on the base-band phase signal but also on the sign of the base-band amplitude signal. This makes a receiver that receives the polar modulated signal more complicated compared with a case in which the base-band amplitude signal is all the time positive or negative.
An inherent problem of polar modulation is the fact that the phase Φ(t) information and the amplitude information R(t) have to be very accurately synchronized with each other in the polar modulated signal P(t) in order to avoid significant distortion and spectral spreading in the polar modulated signal.
In the remainder of this document a signal path via which the amplitude information of the input data signal 109 is transferred to the polar modulated signal 104 is called an A-path (Amplitude information path) and a signal path via which the phase information of the input data signal 109 is transferred to the polar modulated signal 104 is called a P-path (Phase information path).
Real polar modulators include analog blocks and thus the signal propagation delay in both the A-path and the P-path varies according to component and process variation. An analog block can be for example a gain unit or a unit that adds a DC-offset to a signal. Thus it is necessary to include an adjustable delay element in either the A-path or P-path, or both. This is illustrated in FIG. 2. An adjustable delay element 231 in the A-path and an adjustable delay element 232 in the P-path must ensure that a propagation delay on the A-path and a propagation delay on the P-path are sufficiently close to each other. For example, in a GSM-EDGE application the maximum delay mismatch that can be practically tolerated is Ts/128, where Ts is a GSM-EDGE symbol period. In this case the maximum allowable delay mismatch is approximately 29 ns.
Although it is common knowledge that delays on the A-path and on the P-path have to be adjusted with each other, working out when the P-path and the A-path are synchronized is a more difficult task.